The invention relates to an apparatus for measuring foreign substance content in a flowing liquid, with a stream of liquid bypassed for the process through a conduit. The stream is pumped into a cell, and with the cell a beam source is associated whose beam penetrates the cell and is fed to a detector, and the electrical signal of the detector is processed by an electrical circuit to a recognition signal, in accordance with DE No. 37 335 73, to which EP No. 310740 corresponds.
Apparatus are known for measuring the foreign substance content in a gas stream, and are used for process control in chemical plants, for flue gas measurement and for measuring emissions from boiler firing and from motor vehicles. These known apparatus operate on the nondispersive ultraviolet absorption principle of measurement. The specific absorption of radiation by the component being measured serves as the effect that is utilized.
The ultraviolet radiation is produced in a hollow cathode lamp. A rotary shutter divides the beam into two beams that are separated in time, and a beam splitter divides it into two beams separated in space. The beam that is measured is passed through the cell and strikes a receiver. The completely unaffected beam used for comparison strikes the correction receiver.
The electronic processing of these four signals eliminates the influence of nonselective absorptions, such as cell soiling and aging effects in the radiators and receivers.